Electrowetting based liquid lenses are well known and several patents cover their general description and applications (see, for example, patent EP 1 816 491 A1 and patent EP 1 166 157 A1). The electrowetting based liquid lenses described in these patent applications, as in all current commercial applications, are based on the formulation of two phases, namely a non-conductive phase (i.e. an oil) and a conductive phase, the oil and the conductive phase being generally non-miscible and forming a triple interface on an insulating substrate comprising a dielectric material. In patent EP 1 816 491 A1, the oil comprises carbon, silicon or germanium based monomers, oligomers or homopolymers, and the conductive phase is an aqueous formulation based on water, polar organic compounds and salts. In patent EP 1 166 157 A1, the oil comprises alcanes which may be halogenated.
Prior arts have shown that the contact angle of the oil on the insulating substrate in the presence of the conductive phase should be below 20° to provide electrowetting based liquid lenses with acceptable optical power hysteresis (see, for example, patent EP 1 816 491 A1 and Langmuir 2009, 25(11), 6162-6167). However, oil formulations having a contact angle lower than 20° on the insulating substrate in the presence of the conductive phase are often unachievable and result in liquid lenses having high optical power hysteresis and poor optical qualities. As a consequence of physical observation of required contact angles, efforts have been focused on providing liquid formulations of the oil comprising generally 20 weight percent or more of a wetting agent having a contact angle lower than 20° on the insulating substrate. For example, the wetting agents described in patent EP 1 816 491 A1 are hydrophobic molecules such as halogenated alkanes, halogenated aromatic molecules, alkylhalogenated aromatic molecules, aromatic germanium compounds, and aromatic silicium compounds.
Patent US 2006/151754 A1 and patent WO 2010/015691 A1 describe the use of surfactants in the liquid formulations to reduce interfacial tension between the oil and the conductive phase, the reduction of interfacial tension allowing reduction of the driving voltage of the lenses. In patent US 2006/151754 A1, the surfactants described are anionic, cationic or non-ionic molecules. In patent WO 2010/015691 A1, the surfactants described are non-ionic species such as ethoxylated fatty alcohols, polypropylene glycols or polyethylene glycols.
Patent WO 2004/099846 A1 describes a switchable element where the spatial distribution of two immiscible fluids is controlled by the application of a voltage. A “surfactant” is added to at least one of the liquids in order to achieve proper spatial distribution of the liquids. The “surfactant” described in patent WO 2004/099846 A1 may be a hydrophobic small molecule such as pentafluorophenyltrimethylsilane.
However, it is not always possible to design oil formulations with a substantial amount, such as 20 weight percent or more, of a component, such as a wetting agent, having a contact angle lower than 20° on the insulating substrate in the presence of the conductive phase.
Moreover, the prior art compositions comprising a substantial amount of a component having a contact angle below 20° on the insulating substrate may result in electrowetting-based liquid lenses which do not age well. For instance, after application of 40V AC 1 kHz for long times at a high service temperature, some lenses will show a drift of performance, as shown in FIG. 1. Accordingly, there exists a continuing need to provide highly reliable electrowetting optical devices with improved, stable and long-lived optical performances.